This invention relates to lead wires for use with oxygen sensors and especially to lead wires formed from multiple strands made of different materials.
Internal combustion engines and particularly automotive-type internal combustion engines produce exhaust gases which include carbon monoxide, unburned or partially burned hydrocarbons and nitrogen oxides. These materials are undesirable byproducts of the combustion process, and their presence in the exhaust gases can be substantially reduced by proper control of combustion conditions. One condition which is important in establishing efficient combustion and hence reduced levels of pollutants in the exhaust gas is the amount of air provided to the combustion process. The amount of air introduced into the combustion chamber is frequently controlled by systems which first require determining the oxygen content in the exhaust gas. This information is then utilized to control the respective amounts of fuel and air being supplied to the engine so that the exhaust gases will have the desired combustion. Thus, electrochemical sensors have heretofore frequently been used as part of electrical systems in automobiles for measuring and controlling the composition of exhaust gases. One such sensor is disclosed in U.S. Pat. No. 5,290,421 to Reynolds et al, which is hereby incorporated by reference.
Such sensors typically utilize a solid electrolyte to determine the oxygen concentration in the exhaust gases. The electrolyte typically comprises an oxygen-ion-conductive tube or cone having an electrode on the outer and inner surfaces thereof. The outer surface of the sensor is exposed to the exhaust gases, and the interior of the sensor is provided with a reference source of oxygen, such as ambient air. In operation, the differential in oxygen concentration between the exhaust gases and the reference source causes conduction of oxygen ions through the ion-conductive body, resulting in an electrical current which is dependent upon the relative content of oxygen in the exhaust gas and the reference source.
In order to fully activate the solid electrolyte of such sensors and to obtain an appreciable output voltage for measuring oxygen concentration, the sensor element must be heated to an elevated temperature. It has frequently been common practice to rely upon the heat of the exhaust gases passing over the outer electrode to cause the necessary increase in the temperature of the sensor element. However, this procedure has a drawback, namely, such arrangements result in a sensor that is essentially inoperative or only marginally operative, during the warm-up period of the internal combustion engine; yet, it is during this warm-up period that the concentration of pollutants in the exhaust gases is the highest. In order to overcome this disadvantage, oxygen sensors are provided with an electrical heating element for rapidly increasing the temperature of the sensor.
Thus, oxygen sensors require electrically-conductive pathways to carry: (1) the electrical current which is proportional to the oxygen concentration in the exhaust gases in a feedback loop to the control system which determines the fuel/air ratio supplied to the engine; and (2) the electrical current which powers the heating element allowing the oxygen sensor to operate effectively during the transient engine warm-up period.
The conductive pathways are provided by oxygen sensor lead wires. The lead wires are subject to extremely harsh environmental conditions. They must run between the exhaust system of an automobile and the engine compartment and are, thus, subject to extremes of heat, cold, vibration, tensile and compression forces and abuse from roadway hazards, yet they must maintain electrical continuity, ideally for the operational life of the vehicle, to ensure that the signals from the oxygen sensor are communicated to the control system with the utmost fidelity and that the heating element receives the necessary power to maintain the sensor at the required operating temperature during the critical warm-up period of engine operation.
To meet the harsh environmental and performance demands, lead wires for oxygen sensors have developed into multi-strand wires having various strands of different material types redundant to provide the flexibility, robustness, strength and long fatigue life required for effective operation. The conventional wisdom teaches that these characteristics can be best achieved by increasing the number of strands while decreasing the gage of each strand. For example, lead wires having 37 strands are not uncommon, and lead wires having over 100 strands are also in production.
While multi-strand lead wires developed according to the conventional theories do exhibit the characteristics necessary for effective use with oxygen sensors, such lead wires suffer from a tremendous cost disadvantage in that they are complicated, expensive and difficult to produce. Production is expensive because with increasing numbers of strands, it becomes more difficult to lay them together in one pass through the wire laying machines, thus, requiring multiple passes which increase the production time required. Wires having more and finer strands are also more prone to the phenomenon of xe2x80x9cbirdcagingxe2x80x9d a failure mode which occurs during production when the wire is subjected to compression forces and the strands splay outwardly to form a cage-like expansion of a section of the wire. Birdcaging can result in a xe2x80x9chigh strandxe2x80x9d, an individual strand which extends outwardly from the multi-strand wire further than the other strands comprising the wire. The projecting strand often becomes caught on a piece of machinery or a die during production, and the strand is stripped from the wire as the wire passes through the machine, eventually forming a tangled mass of strand and forcing a shutdown of the production line and scrapping of a significant length of the wire produced. The increased propensity for birdcaging also limits the speed at which the wire laying machinery can be operated, in order to keep the forces placed on the wire low and avoid birdcaging or other failures.
Another disadvantage of traditional multi-strand lead wires is that such wires tend to yield and take a permanent set when packaged on a spool or drum. The wire must later be straightened so that it can be attached to the oxygen sensor or other terminals, usually by automated crimping machines. The straightening process adds a step which increases the cost and decreases the rate of production. The straightening process also subjects the wire to potential damage in that the adhesion between the insulating layer and the wire can be disrupted, allowing significant lengths of the insulation to separate from the wire, rendering the wire worthless and, thus, lowering production efficiency.
Yet another disadvantage of traditional multi-strand lead wires is their xe2x80x9cnotch sensitivityxe2x80x9d or lack of toughness in resisting physical damage without developing indentations, cracks or other flaws, usually in the outermost strands comprising the wire. Notch sensitivity is important because any flaws in the wire strands serve as stress risers and crack initiation points from which cracks propagate and cause premature fatigue failure of the strands when the wire is subjected to reverse bending stresses as experienced, for example, in a high vibration environment. As individual strands fail in fatigue, the stress is shared by an ever decreasing number of remaining strands, thus, increasing the stress on the strands and accelerating the fatigue failure of the wire. Multi-strand wires having relatively soft nickel plated copper strands in the outermost layer are particularly notch sensitive. Damage to the wire can hardly be avoided, and can occur during the production process, during installation or in use. Crimping of the wires to form electrical connections can be especially damaging to the outer wire layer and can shorten the fatigue life of the wire dramatically.
Clearly, there is a need for an improved oxygen sensor lead wire which can meet the harsh environmental conditions and performance demands but which is simple and inexpensive to produce.
The invention concerns a lead wire for use with an oxygen sensor. In a preferred embodiment, the lead wire according to the invention comprises an elongate center strand having a relatively high tensile strength. Hard stainless steel is a preferred material for the center strand. A plurality of elongate first strands, each having a relatively high electrical conductance, are arranged circumferentially in spaced relation around the center strand. The first strands preferably comprise a copper alloy. A plurality of elongate second strands, each having a relatively high tensile strength, are arranged circumferentially in spaced relation around the center strand and between the first strands in an alternating pattern, preferably at equal separation angles circumferentially around the center strand. Hard stainless steel is again preferred. A plurality of elongate third strands, each having a relatively high electrical conductance, are arranged circumferentially in a plurality of groups around the first and second strands. Each of the groups comprises a predetermined number of the third strands, preferably three. A plurality of elongate fourth strands, each having a relatively high tensile strength, are arranged circumferentially around the first and second strands. Each of the fourth strands are positioned between two of the groups of the third strands. Preferably, the fourth strands are spaced circumferentially around the center strand at equal separation angles. Quarter-hard stainless steel is the preferred material for the fourth strands. Half-hard stainless steel may be any alternate material.
It is an object of the invention to provide an oxygen sensor lead wire which has a high tensile strength and fatigue life.
It is another object of the invention to provide an oxygen sensor lead wire comprised of a minimum of strands.
It is yet another object of the invention to provide a lead wire with a relatively low notch sensitivity which can resist physical damage and avoid flaws which result in stress risers which cause premature fatigue failure of the wire.
It is again another object of the invention to provide an oxygen sensor lead wire which can be formed in one pass through automated wire laying machinery.
It is yet another object of the invention to provide a lead wire which is less prone to birdcaging failure.
It is still another object of the invention to provide a lead wire which is less prone to the high strand condition and its associated failure.
It is yet another object of the invention to provide a lead wire which allow the wire laying machinery to run at higher speeds.
It is also another object of the invention to provide a lead wire which is less prone to take on a permanent set when wound around a spool or drum.